1. Field of the Invention
The present invention relates to a self-starting synchronized induction motor using a permanent magnet rotor.
The synchronized induction motor of the present invention may be used, for example, in a winding system in a spinning plant.
2. Description of the Prior Art
In the winding system in a spinning plant, since a plurality of synchronized induction motors are arranged in parallel and driven in synchronism, the motors should not be of external starting type but they should be of self-starting type and also they should be able to pull in a large load inertia.
Heretofore, in the electric motor of this type, a special rotor which operates as an induction motor at the start of the motor and can be operated as a synchronous motor while making use of a permanent magnet of the rotor after pull-in has been used.
A structure of the permanent magnet rotor is disclosed, for example, in U.S. Pat. No. 3,967,827. In the rotor of the U.S. Patent, a ring-shaped permanent magnet assembly having magnetic poles on the periphery is fitted to one end of a conventional cage rotor. In this type of synchronized induction motor, when the rotor speed approaches a synchronous speed, the rotor constitutes an induction motor together with a stator. Therefore, no substantial current flows into a secondary conductor and about 80% of total fluxes pass through the rotor of the induction motor while only about 20% of total fluxes pass through the ring permanent magnet. Consequently, it is difficult to attain a synchronized induction motor of large capacity such as 100 watts or higher.
A large capacity synchronized induction motor of 100 watts or higher is disclosed in the Japanese Utility Model Application Laid-Open No. 114,612/76 entitled "A Rotor of a Magnet Type Synchronous Machine". In the rotor of this application, a number of cage secondary conductors are embedded in the vicinity of an outer surface of a rotor core, magnets are incorporated inside the rotor core, and slits for preventing the short of magnetic fluxes of the permanent magnets when the motor is operated as a synchronous motor are radially formed in a magnet housing. In this type of synchronized induction motor, when the rotor speed approaches the synchronous speed, most fluxes do not pass through the slits but pass through the permanent magnet. Consequently, a large capacity motor can be attained and the drawback encountered in the synchronized induction motor disclosed in the U.S. Pat. No. 3,967,827 is overcome.
As described above, this type of synchronized induction motor is used in spinning machines or the like, and in that case, a number of such motors are powered from a power supply such as an inverter having a fixed frequency and driven in synchronism. When such motors are used in spinning machine, the speed of the spinning machines is proportional to a speed of a roll directly coupled to the synchronized induction motors. In order to optimize manufacture it is desirable to increase the speed of the motors. If the speed of the motors is doubled, a yield of the system can be doubled, and for a given yield the scale of the system can be reduced to one half. For this reason, it is required that the synchronized induction motor have enough strength to withstand high speed operation. The structure shown in the Japanese Utility Model Application Laid-Open No. 114,612/76 suffers from a limitation to the above requirement.
Namely, firstly, since the permanent magnets are housed in the core, slits for preventing the short of the magnetic fluxes of the permanent magnet are necessary. Consequently, the strength of the core cannot be high enough and hence the speed of the synchronized induction motor cannot be increased greatly.
Secondly, a problem in increasing the speed resides in the need of increasing a danger speed of the rotor which is a rotational speed corresponding to a primary resonance frequency of a bearing span of a rotor shaft. In order to raise the danger speed of the rotor, a diameter of the rotor shaft need be increased. In this case, a sectional area of the core defined between a starting secondary conductor which, together with a stator, constitute the induction motor and the rotor shaft, that is, a sectional area of core back is decreased. As a result, a magnetic circuit is apt to be saturated at the start of the synchronized induction motor and a starting current increases.